BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an operation of a ballast. The present invention specifically relates to ground fault detectors on the input side of a ballast.

2. Description of the Related Art Currently, an isolated safety circuit on the output side of a ballast are employed to protect a person from shock when installing lamps. For example, U. S. Patent No. 4,563, 719, U. S. Patent No. 4,663, 571, and U. S. Patent No. 4,855, 860 are all directed toward an isolated safety circuit on the output side of a ballast. However, a drawback to this mode of protection is the sensitivity of the circuit to potential shock as related to an activation threshold that may still result in an uncomfortable electric sting being felt by the person of a non-isolated ballast. The present invention is an improvement over the prior art.

SUMMARY OF THE INVENTION The present invention is a ballast safety circuit. Various aspects of the present invention are novel, non-obvious, and provide various advantages. While the actual nature of the present invention covered herein can only be determined with reference to the claims appended hereto, certain features, which are characteristic of the embodiments disclosed herein, are described briefly as follows.

One form of the present invention is a device comprising a ballast and a ballast safety circuit. The ballast has a first input line and a second input line. The ballast safety circuit is in electrical communication with the first input line and the second input line where the ballast safety circuit is operable to detect an occurrence of a leakage current flowing through the second input line when a person is attempting to a mount a lamp on the ballast.

In one embodiment, the ballast safety circuit includes a magnetic core, a first coil winding wound around the magnetic core, a second coil winding wound around the magnetic core, and a third coil winding wound around the magnetic core. The first coil winding is in electric

communication with the first input line. The second coil winding is in electric communication with a second input line of the ballast. The third coil winding is operable to detect the leakage current flowing through the second input line.

The foregoing form as well as other forms, features and advantages of the present invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates one embodiment of a ballast safety circuit in accordance with the present invention; and Fig. 2 illustrates one embodiment of a toroid in accordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS Fig. 1 illustrates a conventional ballast 10 and a ballast safety circuit 20 of the present invention. Ballast 10 includes a pair of diodes D and D2 representative of an input rectification circuit coupled in an input line L. Ballast 10 further includes a pair of capacitors C3 and C4 representative of a DC stable source coupled to an input line N. In normal operation, a capacitor Cl and an inductor Ll resonate to control an operating current Il when a lamp 11 is mounted on the ballast 10 as shown. Additionally, a capacitor C2 and an inductor L2 resonate to control an operating current I2 when a lamp 12 is mounted on the ballast 10 as shown by the dashed version of the lamp 12. A toroid Tl drives an electronic switch Kl and an electronic switch K2 to operate the lamp 11 and the lamp 12.

In a circumstance where the lamp 11 is operating and a person (represented by a resistor R) is attempting to mount lamp 12 as shown, a leakage current (represented by the dashed arrow) can flow through the resistor R to the input line N. The leakage current is typically inadequate to operate lamp 12, but adequate to cause a voltage drop across the resistor R that can be harmful to the person. Ballast safety circuit 20 operates to dissipate the leakage current during a detection of the leakage current to thereby protect the person.

Ballast safety circuit 20 includes a toroid T2, a diode D3, a capacitor C5, and a trip latch 21 in the form of a conventional transistor based circuit. FIG. 2 illustrates one embodiment of toroid T2 having a magnetic core MC, a coil winding L3, a coil winding L4, and a coil winding Ls. Coil winding L3 is coupled to an input line L and is in electric communication with the input rectification circuit (FIG. 1) represented by diode D, and diode D2. Coil winding L4 is coupled to input line N and is in electric communication with the DC stable source (FIG. 1) represented by capacitor C3 and capacitor 4. Coil winding Ls is coupled a diode D3 and is in electric communication with the input rectification circuit, the DC stable source and electronic switch K2 (FIG. 1) via a base node Boni. Diode D3 is coupled to a capacitor Cs and a trip latch 21. Capacitor C5 is further coupled to base node Boni.

Referring to Figs. 1 and 2, in operation, magnetic core MC is prohibited from inducting a normal input current in input line L and input line N, because the induction is opposed. A leakage current, if any, will flow through input line N causing a current differential between input line L and input line N. Coil winding L5 detects the leakage current, and in response thereto, provides a detection signal (not shown) to trip latch 21.

During a reception of the detection signal, trip latch 21 impedes an operation of lamp 12 by latching open electronic switch K2. The result is an expediently dissipation of the leakage current whereby a person mounting lamp 12 does not feel the leakage current.

In one embodiment, the number of turns Nl of coil winding L3 and/or coil winding L4, and the number of turns N2 of coil winding Ls are in accordance with the following equation [1]: N2 = (L * U)/ (N1 * f * A * p * I) [1] L is the length of a magnetic path in the toroid T2, U is a trip voltage to latch open electronic switch K2, f is an exciting current frequency, A is the cross-section of magnetic core MC, u. is the permeability of magnetic core MC, and I is the leakage current.

Exciting current frequency f and leakage current I are measured under the assumption resistor R (FIG. 1) is 500 ohms. Trip voltage U is determined from an operational specification of electronic switch K2. Cross-section A and 11 are determined from the dimensions and material of magnetic core MC. Turns Nl and turns N2 can therefore be selected to maximize the efficiency of toroid T2.

Referring to Fig. 1, ballast 10 was utilized to facilitate an explanation of ballast safety circuit 20 in accordance with the present invention. Those having ordinary skill in the art will appreciate from this explanation that a ballast safety circuit in accordance with the present invention (e. g. , ballast safety circuit 20) can be employed with any type of ballast having at least two input lines. As such, for some types of ballasts, the leakage current may flow from resistor R to the input line L (Fig. 1).

While the embodiments of the present invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the present invention. The scope of the present invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.